Surgical tools employing jaws with substantially parallel faces allow a user to apply a more uniform grasping pressure across tissue than with conventional (angular motion) graspers. The geometry of angular motion graspers causes the base of the jaws to compress tissue much tighter than the tips of the jaws, for a given handle force applied, due to the difference in distance from the fulcrum. When grasper jaws are partially open, angular motion jaws have a much larger space between jaws at the tip than at the base of the jaw, while a parallel jaw has the same width between the base of the jaw and the tip of jaw no matter what distance the jaws are apart. Therefore, if the jaws are full of tissue the parallel jaws would compress the tissue uniformly along its entire length while the angular motion jaws would compress the tissue at the base much more than at the tip.
The uniform distance between jaws in a parallel arrangement allows the user to have a more controlled manipulation of tissue. Depth perception and alignment of tissue with respect to the jaws is a lesser problem because the jaws are the same distance from each other along their entire length. The uniform distance and motion of parallel jaws eliminates any component of a force vector that would push tissue out of the jaws distally during closing. A parallel jaw design also allows for a more uniform coagulation of vessels because of the uniform compression of tissue between the jaws. For a given separation distance at the jaw tips, larger volumes of tissue may be grasped compared to an angular motion grasper.
A parallel jaw grasper will be less likely to become fouled with tissue from a prior coagulation when several coagulations are being performed in a sequence. This is because the jaws can be separated along their full length allowing tissue to be removed from one jaw using a second instrument (or by simply falling away). A conventional instrument tends to create a possibility for small amounts of tissue to become lodged near the proximal end of the jaw area.
However, there are also benefits to grasper configurations with an angular displacement of the jaws at the end of the opening motion. This allows the parallel jaws to get the tips much wider than is possible with a purely parallel motion. The angular displacement of the jaws allows effective grasping of tissue.
It would be beneficial to have a device that is capable of both parallel movement and angular movement in order to take advantage of the benefits associated with both configurations. For example, by adding an angular movement towards the end of the instrument motion, the jaws will more likely fit within a smaller trocar cannula but still open very wide at the tip to aid with dissection and get around larger tissue areas for grasping.